Particulate Silver Powder and Method of Manufacturing Same

ABSTRACT

A particulate silver powder has an average particle diameter measured by TEM observation (DTEM) of 200 nm or less, an aspect ratio of less than 2.50, and a {(DTEM)/(Dx)} of 5.0 or less (where (Dx) denotes X-ray crystallite size). The particulate silver powder has a content of each of I − , Cl − , SO 4   2− , NO 3   −  and CN −  of 100 ppm or less. The particulate silver powder is obtained by subjecting a silver compound other than silver nitrate to reduction in an organic solvent having a boiling point of 85° C. or greater at a temperature of 85° C. or greater and in the presence of an organic protective agent.

TECHNICAL FIELD

The present invention relates to a spherical, fine, particulate powderof silver (especially one of a particle diameter on the nanometer order)and a liquid dispersion thereof, particularly to a particulate silverpowder low in corrosive components that is a suitable interconnectforming material for fabricating fine circuit patterns, especially forforming interconnects by the ink-jet method, and a method ofmanufacturing the powder. The particulate silver powder of the presentinvention is also a suitable material for forming interconnects on LSIsubstrates, and electrodes and interconnects of flat panel displays(FPDs), and for filling in fine trenches, via holes and contact holes.It is also suitable for use as a coloring material for automobile paintsand the like. Moreover, its low impurity content and toxicity level makeit useful as a carrier for adsorbing biochemical substances and the likein the fields of medical treatment, diagnostics and biotechnology.

BACKGROUND ART

When the size of solid substance particles reaches the ultrafinenanometer order (called “nanoparticles” in the following), the specificsurface area of the particles becomes very great, so that, even thoughthey are solids, their interface with a gas or liquid becomes extremelylarge. Their surface characteristics therefore strongly affect theproperties of the solid substance.

It is known that the melting point of metal nanoparticles isdramatically reduced from that in the bulk state. In comparison withconventional micrometer-order particles, therefore, metal nanoparticlesoffer not only fine interconnect formation capability but also otherfeatures such as low-temperature sinter capability. Owing to the lowresistance and excellent weatherability of silver nanoparticles, andalso their low price compared with other noble metal nanoparticles,silver nanoparticles are seen as metal nanoparticles with particularpromise as the next-generation material for fine interconnects.

Known methods of manufacturing nanometer-order particles of silver arebroadly divided into vapor phase methods and liquid phase methods. Vaporphase methods are ordinarily methods that conduct deposition in a gas.Patent Document 1 describes a method of vaporizing silver in a helium orother inert gas atmosphere, under a reduced pressure of around 0.5 torr.Patent Document 2 teaches a liquid phase method for obtaining a silvercolloid by reducing silver ions in an aqueous phase using an amine andtransferring the obtained silver deposition phase to an organic solventphase (polymeric dispersant). Patent Document 3 describes a method inwhich a reducing agent (alkali metal borohydride or ammoniumborohydride) is used to reduce a silver halide in a solvent in thepresence of a thiol type protective agent.

Patent Document 1: JP 2001-35255A

Patent Document 2: JP 11-319538 A

Patent Document 3: JP 2003-253311A

Problems to be Overcome by the Invention

The silver particles obtained by the vapor phase method of PatentDocument 1 are 10 nm or less in diameter and have good dispersibility insolvent. However, the technology requires a special apparatus. Thismakes it difficult to synthesize large quantities of silvernanoparticles for industrial use. In contrast, the liquid phase methodis basically suitable for large-volume synthesis but has a problem inthat the metal nanoparticles in the liquid have a strong tendency toagglomerate, making it difficult to obtain a monodispersed nanoparticlepowder. In most cases, citric acid is used as the dispersant whenmanufacturing metal nanoparticles, and the metal ion concentration inthe liquid is usually very low as 10 mmole/L (0.01 mole/L) or lower.This is a barrier to its industrial application.

Although the foregoing method of Patent Document 2 achieves synthesis ofstably dispersed silver nanoparticles by using a high metal ionconcentration of 0.1 mole/L or greater and a high reaction mixtureconcentration, it suppresses agglomeration by using a polymericdispersant having a high number average molecular weight of several tensof thousands. The use of a dispersant of high molecular weight is not aproblem when the silver nanoparticles are to be used as a coloringmaterial, but when the particles are to be used in circuit fabricationapplications, a firing temperature that is equal to or higher than thepolymer boiling point is required and, in addition, pores readily formin the interconnects after the firing, so that problems of highresistance and breakage arise, making the particles not altogethersuitable for fine interconnect applications.

The foregoing method of Patent Document 3 conducts the reaction at arelatively high reactant concentration of 0.1 mole/L or greater anddisperses the obtained silver particles of 10 nm or less using adispersant. As a suitable dispersant, Patent Document 3 proposes a thioltype dispersant, which can be readily vaporized by low-temperaturefiring at the time of interconnect formation because it has a lowmolecular weight of around 200. However, the thiol type surfactantcontains sulfur (S), and sulfur causes corrosion of interconnects andother electronic components, making it an unsuitable element forinterconnect formation applications. The method is therefore notsuitable for interconnect formation applications.

Moreover, the liquid phase method uses silver nitrate, silver halide orthe like as the silver source material (Patent Documents 2 and 3), sothat the reaction solution unavoidably contains many ions originatingfrom the silver source material, such as I⁻, Cl⁻, SO₄ ²⁻, NO₃ ⁻ and CN⁻.In the case of nanoparticles, specific surface area is extraordinarilylarge, solid-liquid separation and washing are difficult, and thereactivity of the ions with silver is high since most of the ionsoriginating from the starting material originally formed compounds withsilver. Because of these and other factors, the ions originating fromthe starting material are found to be adsorbed on the silver particlesafter reaction or to have reacted with them and become present asinclusions therein. A liquid dispersion prepared using the particlessimilarly contains the ions as impurities. On the other hand, theincreasingly sophisticated performance capabilities of electronicequipment in recent years has placed severe demands on the components ofsuch equipment, and the content of constituents and elements thatdegrade reliability is required to be managed on the ppm order. Undersuch circumstances, it is undesirable for corrosive components like I⁻,Cl⁻, SO₄ ²⁻, NO₃ ⁻ and CN⁻ to be entrained in the liquid dispersion ofthe particulate silver powder.

Therefore, an object of the present invention is to overcome suchproblems by providing a nanoparticle powder of silver low in corrosivecomponents that is suitable for fine interconnect formationapplications, and a liquid dispersion thereof, at low cost and in largequantities. Moreover, since monodispersion of spherical silvernanoparticles of uniform diameter is preferable, another object is toprovide a liquid dispersion of such silver particles.

Means for Solving the Problems

In accordance with the present invention, there is provided aparticulate silver powder having an average particle diameter measuredby TEM observation (DTEM) of 200 nm or less, preferably 100 nm or less,more preferably 30 nm or less, an aspect ratio of less than 2.50, and a{(DTEM)/(Dx)} of 5.0 or less (where (Dx) denotes X-ray crystallitesize), which particulate silver powder has a content of each of I⁻, Cl⁻,SO₄ ²⁻, NO₃ ⁻ and CN⁻ of 100 ppm or less. The particulate silver powderpreferably has a TEM average particle diameter (DTEM) of 100 nm,preferably 30 nm or less, and has adhered to the surfaces of theparticles thereof an organic protective agent (typically a fatty acid oran amine compound) having a molecular weight of 100 to 400.

The present invention further provides a liquid dispersion of silverparticles obtained by dispersing such particulate silver powder in anorganic solvent (typically a non-polar or low-polarity solvent), whichliquid dispersion of silver particles has an average particle diameter(D50) measured by the dynamic light-scattering method of 200 nm or less,a degree of dispersion {(D50)/(DTEM)} of 5.0 or less, and a content ofeach of I⁻, Cl⁻, SO₄ ²⁻, NO₃ ⁻ and CN⁻ of 100 ppm or less.

Further, as a method of manufacturing such a particulate silver powder,the present invention provides a method of manufacturing a particulatesilver powder, wherein a silver compound other than silver nitrate(typically silver carbonate or silver oxide) is reduced in an organicsolvent (typically an alcohol or polyol) having a boiling point of 85°C. or greater at a temperature of 85° C. or greater and in the presenceof an organic protective agent (typically a fatty acid or amino compoundhaving a molecular weight of 100 to 400).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an electron microscope (TEM) photograph of a nanoparticlepowder of silver of the present invention.

FIG. 2 is an electron microscope (TEM) photograph of the nanoparticlepowder of silver of the present invention taken at a differentmagnification from that of FIG. 1.

FIG. 3 is an electron microscope (TEM) photograph of anothernanoparticle powder of silver of the present invention.

FIG. 4 is an electron microscope (TEM) photograph of the othernanoparticle powder of silver of the present invention taken at adifferent magnification from that of FIG. 3.

FIG. 5 is an x-ray diffraction chart of a nanoparticle powder of silverof the present invention.

PREFERRED EMBODIMENTS OF THE INVENTION

The present inventor conducted numerous experiments to manufacturenanoparticle powder of silver by the liquid phase method and found itwas possible to obtain a powder composed of spherical silvernanoparticles of uniform particle diameter by subjecting a silvercompound other than silver nitrate (typically silver carbonate or silveroxide) to reduction treatment at a temperature of 85° C. or greater inan alcohol or polyol having a boiling point of 85° C. or greater (whilerefluxing vaporized alcohol or polyol to the liquid phase) and in theco-presence of an organic protective agent having a molecular weight of100 to 400. This nanoparticle powder of silver disperses well in adispersant because it is in a condition of having the organic protectiveagent adhered to its surface. Moreover, the powder can be consistentlyobtained with a particle diameter on the nanoparticle order of 200 nm orless, preferably 100 nm or less, more preferably 30 nm or less. Inaddition, the powder has a content of each of I⁻, Cl⁻, SO₄ ²⁻, NO₃ ⁻ andCN⁻ of 100 ppm or less and was therefore found capable of serving as amaterial suitable for forming interconnects of fine circuit patterns,especially for forming interconnects by the ink-jet method.

The characteristic features of the invention particulate silver powderare explained individually below.

TEM Particle Diameter (DTEM)

The average particle diameter (DTEM) of the invention particulate silverpowder measured by TEM (transmission electron microscope) observation is200 nm or less, preferably 100 nm or less, more preferably 30 nm orless. It is determined by measuring the diameters of 300 discrete,non-overlapping particles observed in a 600000×TEM image and calculatingthe average thereof. The aspect ratio is determined from the sameobservational results.

Aspect Ratio

The aspect ratio (ratio of long diameter/short diameter) of theparticulate silver powder of this invention is less than 2.0, preferablynot greater than 1.2, more preferably not greater than 1.1. Theparticles in the photograph of FIG. 1 are substantially spherical andhave an (average) aspect ratio of not greater than 1.05. They aretherefore ideal for interconnect formation applications. If the aspectratio of not less than 2.0, particle packing is degraded when theparticle liquid dispersion is applied to a substrate and dried, whichmay cause pores to occur during firing, increasing the resistance andpossibly giving rise to interconnect breakage.

Degree of Single Crystal Grain

Degree of single crystal grain {(DTEM)/(Dx)} is represented by the ratioof TEM particle diameter to X-ray crystallite size. X-ray crystallitesize (Dx) can be determined from the results of x-ray diffractionmeasurement using the Scherrer relationship. It is determined asfollows.

The Scherrer relationship is expressed by the general equation:

D=K·λ/β Cos θ,

where K is Scherrer constant, D is crystallite size, λ is wavelength ofthe x-ray used for the measurement, β is half-value width of the x-raydiffraction peak, and θ is Bragg angle of the diffraction line.

If 0.94 is used as the value of K and a Cu X-ray tube is used, theequation can be rewritten as:

D=0.94×1.5405/β Cos 74.

Degree of single crystal grain is represented by the ratio of TEMparticle diameter to X-ray crystallite size {(DTEM)/(Dx)}. The degree ofsingle crystal grain approximately corresponds to the number of crystalsper particle. The higher the value of the degree of single crystalgrain, the greater is the number of crystallites, which called theparticle to be composed of multiple crystals. The degree of singlecrystal grain of the invention silver particles is 5.0 or less,preferably 2.0 or less, more preferably 1.0 or less. Grain boundarieswithin the particles are therefore few. Electrical resistance rises withincreasing number of grain boundaries present. In the inventionparticulate silver powder, the value of the degree of single crystalgrain is low, which gives it low resistance and makes it suitable foruse in conductors.

Average Particle Diameter by the Dynamic Light-Scattering Method

The invention liquid dispersion obtained by mixing the particulatesilver powder and the organic solvent has an average particle diameter(D50) by the dynamic light-scattering method of 200 nm or less and thedegree of dispersion {(D50)/(DTEM)} is 5.0 or less.

The particulate silver powder of this invention readily disperses in thedispersion medium and can remain in a stable dispersed state in thedispersion medium. The dynamic light-scattering method can be used toassess the state of dispersion of the silver nanoparticles in thedispersion medium, and also to calculate the average particle diameter.The principle of the method is as follows. In a liquid, thetranslational, rotational and other forms of Brownian movement ofparticles having a diameter in the range of around 1 nm to 5 μmordinarily changes the location and orientation of the particles frominstant to instant. When a laser beam is projected onto the particlesand the scattered light that emerges is detected, there are observedfluctuations in the scattered light intensity that are attributable tothe Brownian movement. By measuring the time dependence of the scatteredlight intensity, it is possible to determine the velocity of theBrownian movement (diffusion coefficient) of the particles and also tolearn the size of the particles. If the average diameter of theparticles in the dispersion medium measured utilizing this principle isclose to the average particle diameter obtained by TEM observation, thismeans that the particles in the liquid are individually dispersed (notattached to each other or agglomerated). In other words, the particlesin the dispersion medium are spaced apart from each other and thus in astate that enables them to move independently.

The average particle diameter of the particulate silver powder in theinvention liquid dispersion determined by carrying out the dynamiclight-scattering method thereon is on a level not much different fromthe average particle diameter found by TEM observation. Morespecifically, the average particle diameter of the invention liquiddispersion measured by the dynamic light-scattering method is 200 nm orless, preferably 100 nm or less, and more preferably 30 nm or less,which is not very different from the average particle diameter by TEMobservation. This means that, a monodispersed state is achieved andindicates that the present invention is capable of providing a liquiddispersion in which the nanoparticles of the silver powder areindependently dispersed.

Still, even when the particles are completely monodispersed in thedispersion medium, cases arise in which measurement error or the likecauses differences to arise with respect to the average particlediameter obtained by TEM observation. For example, the concentration ofthe liquid dispersion during the measurement must be suitable for theperformance and the scattered light detection system of the measurementapparatus, and errors occur if the concentration does not ensuretransmission of enough light. Moreover, the signal obtained whenmeasuring nanometer-order particles is so feeble that contaminants anddust come to have a strong effect that may cause errors. Care istherefore necessary regarding pretreatment of samples and thecleanliness of the measurement environment. The laser beam source usedfor nanometer-order particle measurement should have an output power of100 mW or greater so as to ensure adequate scattered light intensity. Inaddition, it is known that when the dispersion medium is adsorbed on theparticles, the adsorbed dispersion medium layer has an effect that worksto increase particle diameter even when the particles are completelydispersed. This effect becomes particularly manifest when particlediameter falls below 10 nm. So it can be assumed that good dispersion ismaintained even if the dispersed particles do not have exactly the samedegree of dispersion as the value found by the TEM observation, providedthat the degree of dispersion (D50)/(DTEM) is 5.0 or less, preferably3.0 or less.

The particulate silver powder of the present invention contains each ofthe ions I⁻, Cl⁻, SO₄ ²⁻, NO₃ ⁻ and CN⁻ at 100 ppm or less. Therefore,so long as a high-purity dispersion medium is used, the liquiddispersion prepared by dispersing the powder in the dispersion mediumcan also have a content of each of the ions I⁻, Cl⁻, SO₄ ²⁻, NO₃ ⁻ andCN⁻ of 100 ppm or less. Dispersion mediums suitable for use in thepresent invention include, for example, non-polar and low-polarityorganic dispersants such as hexane, toluene, kerosene, decane, dodecaneor tetradecane.

Manufacturing Method

The particulate silver powder of the present invention can bemanufactured by subjecting a silver salt other than silver nitrate(typically silver carbonate or silver oxide) to reduction treatment at atemperature of 85° C. or greater in an alcohol or polyol having aboiling point of 85° C. or greater and in the co-presence of an organicprotective agent.

The alcohol or polyol used in the present invention as an organicsolvent/reducing agent is required to have a boiling point of 85° C. orgreater but is not otherwise particularly limited. An alcohol having aboiling point below 85° C. does not readily enable a reactiontemperature of 85° C. or higher without using a special reactor such asan autoclave. Complete reduction of silver carbonate or silver oxide tosilver is difficult to achieve at a reaction temperature below 85° C.Preferred examples of the organic solvent/reducing agent are isobutanoland n-butanol, either individually or in a mixture of the two, but thisis not a restriction and any organic solvent/reducing agent having aboiling point of 85° C. or greater can be used. As the compound otherthan silver nitrate, a silver compound that is sparingly soluble in theorganic solvent, such as silver carbonate, silver oxide or the like isused. Silver nitrate is undesirable because the starting materialthereof readily entrains I⁻, Cl⁻, SO₄ ²⁻, NO₃ ⁻ and CN⁻ and other suchions. Silver carbonate, silver oxide and the like are used in the formof powder.

As the organic protective agent, there is used a metal coordinationcompound of a molecular weight of 100 to 400 that has coordinatingcapability toward silver, typically a fatty acid or amino compound. Whena compound having no or only weak coordinating capability toward silveris used, a large amount of the protective agent is required to producenanoparticles. Viewed generally, metal coordination compounds includeisonitrile compounds, sulfur compounds, amino compounds, and fatty acidshaving a carboxyl group. However, a sulfur compound would degrade thereliability of electronic components because it would cause corrosionowing to the sulfur it contains. Isonitrile compounds have problems oftoxicity and the like. This invention enables provision of a materialfor forming interconnects free of these problems by using a fatty acidor amino compound having a molecular weight of 100 to 400 as the organicprotective agent.

The amino compound is preferably a primary amine. A secondary amine ortertiary amine would itself operate as a reducing agent, which wouldcause an inconvenience in a case where an alcohol is already in use as areducing agent, because the resulting presence of two types of reducingagent would complicate control of the reduction rate and the like. Anamino compound or fatty acid having a molecular weight of less than 100has low agglomeration suppressing effect. One with a molecular weightexceeding 400 has strong agglomeration suppressing effect but also has ahigh boiling point. If a particulate silver powder whose particlesurfaces are coated therewith should be used as a material for forminginterconnects, the amino compound or fatty acid would act as a sinterinhibitor during firing. The resistance of the interconnects wouldtherefore become high, possibly to the point of impairing conductivity.Since this is undesirable, it is best to use an amino compound or fattyacid having a molecular weight of 100-400.

The reduction of the silver carbonate or silver oxide in alcohol orpolyol is best conducted using an apparatus equipped with a refluxcondenser to return vaporized alcohol or polyol to the liquid phaseduring the reaction. The silver concentration of the reaction mixture ispreferably made 50 mmole/L or greater. A lower concentration than thisis not suitable because cost increases.

Upon completion of the reaction, the slurry obtained is centrifuged toseparate solid from liquid. The resulting sediment is added with adispersion medium such as ethanol and dispersed therein using anultrasonic disperser. The dispersion is again centrifuged and thesediment is again added with ethanol and dispersed with the ultrasonicdisperser. This process of [solid-liquid separation dispersion] isrepeated three times, whereafter the supernatant is discarded and thesediment dried to obtain the particulate silver powder of the presentinvention.

The particulate silver powder obtained is dried in a vacuum drier (for12 hours at 200° C., for example), whereafter the dry product can beassayed for silver purity using the gravimetric method (upon dissolutionin nitric acid followed by addition of HCl to prepare a silver chlorideprecipitate) to measure purity from the weight thereof. The purity ofthe particulate silver powder of the present invention is 90% orgreater. Moreover, as pointed out earlier, the particulate silver powderhas a content of each of I⁻, Cl⁻, SO₄ ²⁻, NO₃ ⁻ and CN⁻ of 100 ppm orless. As the dispersion medium for dispersing the invention particulatesilver powder can be used an ordinary non-polar solvent or alow-polarity solvent such as hexane, toluene, kerosene, decane, dodecaneor tetradecane. The liquid dispersion obtained is then centrifuged toremove coarse particles and agglomerated particles. Next, only thesupernatant is collected as a sample to be subjected to TEM, x-ray,particle size distribution and other measurements.

WORKING EXAMPLES Example 1

Isobutanol (reagent grade from of Wako Pure Chemical Industries, Ltd.),200 mL, used as a solvent/reducing agent, was added with 0.1329 mL ofoleic acid (Wako Pure Chemical Industries, Ltd.) and 5.571 g of silvercarbonate powder (Kojundo Chemical Laboratory Co., Ltd.), and themixture was stirred lightly with a spoon. The solution was transferredto a container equipped with a reflux condenser which was then placed inan oil bath. The solution was stirred with a magnetic stirrer at 200 rpmand heated while nitrogen gas used as an inert gas was blown into thecontainer at the rate of 400 mL/min. Refluxing was continued for 3 hoursat 100° C. to complete the reaction. The temperature increase rate to100° C. was 1° C./min.

After the reaction, the slurry was subjected to solid-liquid separationand washing by the procedure set out below:

1. The slurry following the reaction was centrifuged at 5000 rpm for 60minutes in a CF7D2 centrifuge made by Hitachi Koki Co., Ltd. to separatesolid from liquid, and the supernatant was discarded. 2. The sedimentwas added with ethanol and dispersed using an ultrasonic disperser. 3.Steps 1 and 2 were repeated 3 times. 4. Step 1 was performed and thesupernatant was discarded to obtain a sediment.

The pasty sediment obtained in Step 4 was prepared for evaluation asfollows:

a) For the measurement of particle size distribution by TEM observationand dynamic light-scattering, the sediment was added with and dispersedin kerosene, the liquid dispersion was centrifuged to sediment coarseparticles, and the supernatant was recovered to obtain a dispersion ofindependent fine particles of silver.b) For the x-ray diffraction and crystallite size measurement, theindependent liquid dispersion was concentrated to a paste that wascoated onto a non-reflective substrate and analyzed with an x-raydiffractometer.c) For determining Ag purity and yield, the sediment obtained in 4 wasdried in a vacuum drier for 12 hours at 200° C. and the weight of thedried product was measured. More specifically, the Ag purity of thedried product was measured by the gravimetric method (method ofdissolving the dried product in nitric acid, adding HCl to the solutionto prepare a silver chloride precipitate, and measuring the purity fromthe weight thereof). Yield was calculated as (weight of driedproduct/theoretical yield calculated from materials used to prepare thereaction solution)×100(%).d) For measuring the content of the corrosive components I⁻, Cl⁻, SO₄²⁻, NO₃ ⁻ and CN⁻, analysis was performed on the sediment obtained in 4after it had been dried in a vacuum drier at room temperature for 12hours.

The results of the measurements were. TEM average particle diameter: 7.7nm, aspect ratio: 1.2, X-ray crystallite size (Dx): 8.12 nm, and degreeof single crystal grain (DTEM/Dx): 0.95. Only peaks attributable tosilver were observed in the x-ray diffraction results. D50 measured bythe dynamic light-scattering method (Microtrack UPA) was 20.2 μm.D50/DTEM was 2.6. Silver purity was 93% and silver yield was 93.1%. Thecontent of each of I⁻, Cl⁻, SO₄ ²⁻, NO₃ ⁻ and CN⁻ was 100 ppm or less inboth the dried silver nanoparticles and the liquid dispersion.

FIGS. 1 and 2 are TEM photographs of the nanoparticle silver powder ofthis Example (photographs for determining TEM average particle diameterand the like). In these photographs, spherical silver nanoparticles areobserved to be well dispersed at regular intervals. Although a verysmall number of overlapped particles are observed, the measurements ofaverage particle diameter (DTEM), aspect ratio and CV value were madewith respect to completely dispersed particles. FIG. 5 is an x-raydiffraction chart of a nanoparticle silver powder of this Example. Ascan seen in FIG. 5, only peaks attributable to silver are observed.

Example 2

Example 1 was repeated except that 5.571 g of silver carbonate powderwas changed to 4.682 g of silver oxide powder (Dowa Mining Co., Ltd.)The results of the measurements were TEM average particle diameter: 6.0nm, aspect ratio: 1.15, X-ray crystallite size (Dx): 6.52 nm, and degreeof single crystal grain (DTEM/Dx): 0.92. D50 measured by the dynamiclight-scattering method (Microtrack UPA) was 21.2 nm. D50/DTEM was 3.53.Silver purity was 92% and silver yield was 90.1%. TEM photographs of thenanoparticle silver powder are shown in FIGS. 3 and 4. Also for thenanoparticle silver powder of this Example, only peaks attributable tosilver were observed in the x-ray diffraction results. The content ofeach of I⁻, Cl⁻, SO₄ ²⁻, NO₃ ⁻ and CN⁻ was 100 ppm or less in both thedried silver nanoparticles and the liquid dispersion.

Comparative Example 1

An attempt was made to repeat Example 1 with the silver carbonatepowder, 5.571 g, changed to silver nitrate crystal, 6.863 g. The silvernitrate was dissolved in the oleic acid and added to the isobutanol.Although the result was continuously stirred, it did not dissolve. Thesediment obtained following heating and reaction in this condition wassubjected to x-ray measurement but almost no peaks attributable tosilver were observed. Silver oleate was probably produced.

1. A particulate silver powder having an average particle diametermeasured by TEM observation (DTEM) of 200 nm or less, an aspect ratio ofless than 2.50, and a {(DTEM)/(Dx)} of 5.0 or less (where (Dx) denotesX-ray crystallite size), which particulate silver powder has a contentof each of I⁻, Cl⁻, SO₄ ²⁻, NO₃ ⁻ and CN⁻ of 100 ppm or less.
 2. Theparticulate silver powder according to claim 1, whose surface is coatedwith an organic protective agent having a molecular weight of 100 to400.
 3. The particulate silver powder according to claim 2, wherein theorganic protective agent is a fatty acid or an amino compound.
 4. Aliquid dispersion of silver particles obtained by dispersing theparticulate silver powder of claim 1 in a dispersion medium, whichliquid dispersion of silver particles has an average particle diameter(D50) measured by the dynamic light-scattering method of 200 nm or less,a degree of dispersion {(D50)/(DTEM)} of 5.0 or less, and a content ofeach of I⁻, Cl⁻, SO₄ ²⁻, NO₃ ⁻ and CN⁻ of 100 ppm or less.
 5. The liquiddispersion of silver particles according to claim 4, wherein thedispersion medium is a non-polar or low-polarity organic solvent.
 6. Amethod of manufacturing a particulate silver powder which comprises:subjecting a silver compound other than silver nitrate to reduction inan organic solvent having a boiling point of 85° C. or greater at atemperature of 85° C. or greater and in the presence of an organicprotective agent.
 7. A method of manufacturing a particulate silverpowder according to claim 6, wherein the organic solvent is an alcoholor a polyol.
 8. A method of manufacturing a particulate silver powderaccording to claim 6, wherein the silver compound other than silvernitrate is silver carbonate or silver oxide.
 9. A method ofmanufacturing a particulate silver powder according to claim 6, whereinthe organic protective agent is a fatty acid or amino compound having amolecular weight of 100 to
 400. 10. A method of manufacturing a liquiddispersion of silver particles which comprises: dispersing theparticulate silver powder of claim 1 in a non-polar or low-polarityorganic solvent having a boiling point of 85° C. or greater.
 11. Aliquid dispersion of silver particles obtained by dispersing theparticulate silver powder of claim 2 in a dispersion medium, whichliquid dispersion of silver particles has an average particle diameter(D50) measured by the dynamic light-scattering method of 200 nm or less,a degree of dispersion {(D50)/(DTEM)} of 5.0 or less, and a content ofeach of I⁻, Cl⁻, SO₄ ²⁻, NO₃ ⁻ and CN⁻ of 100 ppm or less.
 12. A methodof manufacturing a particulate silver powder according to claim 7,wherein the silver compound other than silver nitrate is silvercarbonate or silver oxide.
 13. A method of manufacturing a particulatesilver powder according to claim 7, wherein the organic protective agentis a fatty acid or amino compound having a molecular weight of 100 to400.
 14. A method of manufacturing a particulate silver powder accordingto claim 8, wherein the organic protective agent is a fatty acid oramino compound having a molecular weight of 100 to
 400. 15. A method ofmanufacturing a liquid dispersion of silver particles which comprises:dispersing the particulate silver powder of claim 2 in a non-polar orlow-polarity organic solvent having a boiling point of 85° C. orgreater.